Specify concrete Python version in README
Specify concrete Python version in README

PiperOrigin-RevId: 398718160
Change-Id: I76ab88d23927f03f2ad149be4f14d6ffa99122b6
GitOrigin-RevId: d4261ba191b1d84917e991148819bf1cf4750e84

deepmind/xmanager,deepmind/xmanager